I live near Hollywood and I can tell you firsthand that it will not be easy. There are many beautiful people out here struggling to make it in this town, and many more who have been broken by it. But don't give up on your dream... You should never give up on your dreams. The point is that you are willing to put in the hard work. My suggestion to you is to get your education first. Finish school. You are in England so you can get into some of the greatest and oldest drama and acting programs in the world. Think how unique and how much it will set you apart if you have a degree from say Oxford. If you are smart and confident, LA and Hollywood will not break you down. Be strong and confident and do not get involved in the hype and party scene, or with drugs. You need to be very mature and confident in yourself.
Acting auditions are brutal. The people there will tear you apart from the way you look to the way you are, and if you are not ready it will really get you down. And all this is only if you get the audition to begin with ... you first need to find an agent who will then refer to you auditions. If you want to make it, work on your self-esteem. Your low self-esteem will simply not do. You will need really high self-esteem so that you do not care what others think and you do not let their opinion rule your life. You also will need something to fall back on... if you complete your education you will be that much more prepared. It will also help in finding a job that will support you until you catch your break. Trust me, you do not want to be a waitress.... it may take years before you get acting work, and the last thing you want is to get stuck in a dead end job.
Finish your education and you will be smart enough to handle anything the acting industry throws at you. Actress Eva Longoria, for example, got a degree before she embarked on her career. You need to be very wary. There are many who will try to take advantage of you, especially if you are a young girl so the more prepared you are the better. Do research, and read up on success stories. Then set up some realistic short term and long term goals (like don't expect to be cast in a hit tv show immediately. Start with a school play or an acting society).
I am not saying all this to discourage but rather to encourage your passion. Yes you are young, but that does not mean you cannot have aspirations. I can say, however, do not close yourself off to other things. Be flexible and take advantage of the opportunities given to you, even if they do not exactly fit in with your acting. Films/tv will be around for a good while so you got time. ;) If you have work as model, seize the opportunity (Tippi Hedron started off as a model). You can save/invest the money and take advantage of all the benefits. Travel and gain a new perspective.
I think if you have some sort of plan devised or at least you can show that you have seriously researched and educated yourself on becoming an actress, friends/family/teachers will not think it stupid. You may not ever make the fame of Angelina Jolie or Cameron Diaz, but you should always do what yoy love. You hope for the best and plan for the worst. I know many entertainment lawyers who are still trying to become actors, even though they are earning 6 figures as attorneys. If this is what you truly want to do, then don't give up on it. Just don't lose yourself in it, and always maintain a very clear picture of who you are. There are so many fake things and fake people in this industry, you need to be able to distinguish the fake from the real.
I would be happy to send you information on California universities, if you are interested. Please feel free to ask me any questions. I hope this helped. Sorry for the long response :) Good luck... hope to hear back from you.